1. Field of the Invention
The present invention is directed to a reset device, and more particularly, to a reset device having a variable threshold to prevent unwanted resets of systems incorporating the reset device, such as systems having rechargeable batteries like wireless phones and portable computers.
2. Discussion of the Prior Art
In systems having controllers, such as micro-processors or digital signal processors (DSPs), the controller resets the system ON and OFF depending on the system""s power input voltage level. For a system having a rechargeable battery and a controller, such as wireless telephones or portable computers, the controller is reset to turn OFF the system, or switch it to a dormant mode where minimal current is consumed, when the battery voltage drops below a predetermined threshold level. A comparator is used to detect when the battery voltage crosses the predetermined threshold voltage level, thus detecting whether the battery voltage is above or below the predetermined threshold.
The controller also turns ON the system when the battery voltage increases above this predetermined threshold level. When the battery voltage momentary drops below a certain level and then returns to normal, referred to as battery re-bound the controller turns OFF and ON, i.e., resets the system. In certain cases, a reset is not desirable when a re-bound of the battery voltage occurs.
One case where a reset is not desirable occurs when the voltage of a rechargeable battery, such as a NiCad battery, re-bounds due to shut-off of a load. Illustratively, the load is a wireless telephone, which is shut-off by the controller since the battery voltage drops below the threshold value. Normally, the voltage of a three cell NiCad Battery is about 3.8 Volts (V). During battery discharge, the voltage drops from approximately 3.8 V to approximately 3.3 V. At the 3.3 V level, the battery is almost fully discharged.
Rechargeable batteries typically suffer permanent damage when fully discharged. To prevent permanent battery damage, it is recommended that near the fully discharged point, e.g., 3.3 V, the load should be turned OFF to prevent further or deep/full discharge of the battery. When the load is turned OFF, the voltage of the almost fully discharged battery slowly re-bounds from approximately 3.3 V to approximately 3.75 Vxcx9c3.80 V, i.e., to approximately the normal battery voltage. Since the voltage level of the battery with the load turned OFF re-bounds to about 3.8 V, which is also the normal battery voltage, the micro-processor will restart and turn ON the system.
As a result of turning ON the system or wireless telephone, the current draw increases. Since the battery is discharged and cannot deliver much current, the battery voltage drops rapidly and the system resets or shuts OFF again. This process repeats itself with increasingly shorter periods of time, causing power oscillation, which is dangerous to the system, and can cause permanent corruption of the system memory, such as an electrically erasable programmable read only memory (EEPROM).
To solve the problem of power oscillation due to battery re-bound resulting from system shut-off, a flip flop is used to exploit a hysteresis effect of the comparator and prevent the power oscillation. However, the flip flop suffers from a number of disadvantages, such as increased cost and reduced response time. In particular, the flip flop has an inherent memory. Thus, when power is removed, the flip flop can remember its last state for a certain period of time.
Further, when the flip flop is first turned ON, it could be in a high or low state. This state of the newly turned ON flip flop is random and unpredictable. Accordingly, the flip flop must first be reset to a desirable initial condition. Using the flip flop not only increases cost, but also reduces response time due to the need for initially resetting the flip flop.
Accordingly, there is a need for a reset device which prevents power oscillation and system damage, is quick and cost effective, and has no inherent memory.
The object of the present invention is to provide a reset device which eliminates the problems of conventional reset devices.
Another object of the present invention is to provide a reset device which prevents power oscillation and system damage.
A further object of the present invention is to provide a reset device which has no inherent memory.
A still further object of the present invention is to provide a reset device which has a quick response time and is cost effective.
The present invention accomplishes the above and other objects by providing a reset device which may be used in electrical devices such as portable telephones. The reset device includes a source, such as a rechargeable battery, which provides an input voltage and a threshold voltage. The reset device also has a comparator with a first input which receives the input voltage and a second input which receives the threshold voltage. A voltage changing device varies the threshold voltage in response to the output level of the comparator and a rate of increase of the input voltage.
Illustratively, the voltage changing device includes a capacitor which delays an increase of the threshold voltage when the input voltage increases. Further, the voltage changing device includes a positive feedback resistor for increasing the threshold voltage when the output level increases. Thus, the positive feedback resistor allows switching the threshold voltage from a first threshold level to a second threshold level when the output level switches from a low level to a high level.
The reset device further includes a regulator which regulates the input voltage to provide the threshold voltage. In addition, a switch is connected to an output of the comparator for providing a reset signal to a load. Illustratively, a controller receives the reset signal and resets the load, e.g., the portable phone. In response to the reset signal, the controller switches the portable phone between a dormant mode and an active mode. In addition, the controller allows trickle charging of the rechargeable battery in the dormant mode and rapid charging in the active mode.